I don't know how many characters that Vietnamese have, but I found this solution: modify the font. Replace the characters you won't use in the font with the Vietnamese characters. Then use those characters in the script.
Ex: # = Ã§, Â½ = ÅŸ
Ã‡ekilin! = #ekilin!
Åžerefsiz = %erefsiz
I did the same for the Turkish characters, and therefore I don't need to use rlBabel.